Question

Polly is pushing a box to the right across the floor with a force of 30 N. The force of gravity is 8 N, and the normal force is 8 N. Which value could describe the force of friction if Polly could not move the box?

Answer 1

If the box doesn't move, horizontal forces balance. Friction must cancel Polly's 30 N push, so the frictional force is 30 N to the left (magnitude 30 N, opposite Polly's push).